Image sensors have become ubiquitous. They are widely used in digital still camera, cellular phones, security cameras, as well as, medical, automobile, and other applications. The technology used to manufacture image sensors, and in particular, complementary metal-oxide semiconductor (“CMOS”) image sensors (“CIS”), has continued to advance at great pace. For example, the demands of higher resolution and lower power consumption have encouraged the further miniaturization and integration of these image sensors.
As pixel cells decrease, so does the size of the photodiodes included within them. Smaller photodiodes results in the reduction of the dynamic range of the CIS and a degradation of image quality. One approach to enhance the performance of small photodiodes is to increase the impurity concentration of the sensor region of a photodiode. However, increasing the impurity concentration of the sensing region of a photodiode tends to cause undesirable effects, such as an increase in image lag.
The dynamic range of a CIS is the ratio of maximum and minimum light intensities that an image sensor can capture. The full well capacity of each photodiode in an image sensor limits the image sensor's ability to capture bright and dark subjects at the same time. As a result, if a long exposure is used to capture dark subjects, bright subjects can lose contrast and become a bright spot. If a short exposure is used to capture bright subjects, dark subjects could blend into a dark background and simply disappear.
Conventional techniques of obtaining one overlaid image from two images captured independently and combining or referencing one onto another include using beam combiners and other optical devices to combine two optical images into one optical path of incident light, which is exposed to a single image sensor. Other methods of obtaining one overlaid image includes using a camera module with a double-sided image sensor chip fabricated by adhering two front-side illuminated (“FSI”) image sensor chips together so that the pixel arrays are facing opposite directions. In this way, two images originating from independent sources can be imagined simultaneously, the images could be processed to obtain one overlaid image.
The former conventional technique introduces optical errors from the beam combiners and other optical devices. The latter conventional technique using back-to-back bonded FSI image sensor chips is bulky, which increases manufacturing costs. With the latter technique, the two images must be precisely aligned through signal processing, often done independently after acquiring each image, and the use of two FSI image sensor chips to obtain one image doubles power consumption and manufacturing costs of the camera module.